The present invention relates to a scanning optical system, and more particularly, to a scanning optical system utilizing a rotating polygonal mirror for deflecting a light beam onto an object surface to be scanned.
Generally, a scanning optical system utilized in a laser imaging device, such as a laser photo-plotter, includes a laser source, a rotating polygonal mirror for deflecting the laser beam emitted from the laser source, and an fxcex8 lens system or imaging optical system, which is arranged between the polygonal mirror and an object surface, for converging the laser beam deflected by the polygonal mirror on the object surface. The polygonal mirror is provided with a plurality of reflecting surfaces and is rotated about a rotation axis at a constant angular velocity. The laser beam is deflected by each reflecting surface, transmitted through the fxcex8 lens system, and converged by the fxcex8 lens system to form a beam spot on the object surface. The beam spot moves on the object surface in a main scanning direction at a constant velocity as the polygonal mirror rotates.
Generally, the polygonal mirror is arranged such that its rotation axis is perpendicular to a main scanning plane, which is parallel to the main scanning direction and includes the optical axis of the fxcex8 lens system. The laser beam emitted from the laser source, impinges on a reflecting surface of the polygonal mirror along the main scanning plane.
It is well known that, in the scanning optical systems as mentioned above, a ghost image of a very low intensity appears on the object surface when the laser beam scans thereon. Such a ghost image appears since a part of the light of the beam spot is scattered and reflected by the object surface, transmitted through the fxcex8 lens system, reflected by another reflecting surface (i.e., a surface next to the one contributing to the beam spot) of the polygonal mirror, and impinges on the object surface after being transmitted through the fxcex8 lens again.
Although the intensity is relatively low, the ghost image still degrades the quality of the image formed on the object surface since it stays at a substantially fixed location on the object surface, and supplies sufficient amount of light for exposure.
Japanese patent publication of examined application HEI 3-5562 discloses a scanning optical system that prevents the ghost image from appearing on the object surface. The scanning optical system according to the above mentioned publication determines the angle between the laser beam, which proceeds along the main scanning plane and impinges on the reflecting surface of the polygonal mirror, and the optical axis of the imaging optical system. The angle is determined such that the ghost image is positioned out of a scanning area where the laser beam is to be scanned based on an equation which defines the angle as a function of the number of the reflecting surfaces of the polygonal mirror, the scanning width on the object surface, and the focal length of the imaging optical system.
However, according to the equation indicated in the publication HEI 3-5562, the angle between the laser beam and the optical axis of the imaging optical system is in inverse proportion to the number of the reflecting surfaces of the polygonal mirror. Therefore, the angle becomes quite small if the number of the reflecting surfaces is increased in order to increase the scanning speed. Accordingly, it is difficult to produce the conventional scanning optical system when the number of reflecting surfaces is too large such that the laser beam incident on the polygonal mirror overlaps the scanning area of the laser beam deflected by the polygonal mirror.
HEI 3-5562 also discloses a scanning optical system that prevents the ghost image from appearing on the object surface by inclining the laser beam incident on the polygonal mirror with respect to the main scanning plane. The scanning optical system is provided with a lens system that consists of lenses having rotationally symmetrical surfaces with respect to the optical axis. The laser beam incident on the polygonal mirror is inclined with respect to the main scanning plane, without changing the position on the reflecting surface on which the laser beam impinges, such that the ghost image appears on the object surface apart from a scan line in the auxiliary scanning direction. Here, the scan line is a line along which a beam spot, formed by the laser beam on the object surface, moves. The ghost image is prevented from appearing on the object surface by arranging a light intercepting member on the passage of light that forms the ghost image on the object surface.
The scan line, however, bows by the above mentioned scanning optical system due to the inclination of the laser beam incident on the polygonal mirror. Such bowing of the scan line are undesirable since it significantly degrades the quality of the image formed on the object surface.
It is therefore an object of the present invention to provide an improved scanning optical system that prevents degradation of an image formed on the object surface due to a ghost image even when the polygonal mirror has a large number of reflecting surfaces, without having a scan line bow.
For the above object, according to one aspect of the present invention, there is provided a scanning optical system for scanning a light beam on an object surface in a main scanning direction, which includes a light source, a polygonal mirror, and a scanning lens system.
The polygonal mirror has a plurality of reflecting surfaces that are arranged parallel to a rotation axis about which the polygonal mirror rotates. The reflecting surfaces deflects a light beam emitted form the light source toward the object surface to scan the light beam in the main scanning direction as the polygonal mirror rotates. The scanning lens system converges the light beam deflected by the reflecting surfaces on the object surface to form a beam spot thereon. A focal length of said scanning lens system in a main scanning direction is the same as a focal length of the scanning lens system in an auxiliary scanning direction. The auxiliary scanning direction is a direction perpendicular to both the main scanning direction and an optical axis of the scanning lens system.
A projection of the light beam incident on the reflecting surfaces onto an auxiliary scanning plane, which is perpendicular to the main scanning direction, inclines against the optical axis in a first direction. Further, a projection of the rotation axis of the polygonal mirror onto the auxiliary scanning plane inclines against the auxiliary direction in a second direction which is opposite to the first direction.
By the scanning optical system constituted as above, the light, reflected by the object surface at the beam spot, is incident on the reflecting surface of the polygonal mirror not perpendicularly but inclined in the auxiliary direction. Due to the inclination, the light is reflected by the reflecting surface toward the object surface and impinges on the object surface at a location apart from the beam spot in the auxiliary direction. Thus, the scanning optical system can separate the ghost image from the beam spot in the auxiliary direction.
Further, a bow of a scan line, along which the beam spot moves as the polygonal mirror rotates, can be reduced since a bow due to the inclination of the light beam incident on the reflecting surface of the polygonal mirror and a bow due to the inclination of the rotation axis of the polygonal mirror occur in opposite directions and cancels each other.
According to another aspect of the invention, the scanning optical system may be configured such that a projection of the light beam incident on the reflecting surfaces onto an auxiliary scanning plane, which is perpendicular to the main scanning direction, and a projection of the rotation axis of the polygonal mirror onto the auxiliary scanning plane are inclined, respectively, against the optical axis and a auxiliary direction such that a part of a light of the beam spot is reflected by the object surface, further reflected by the reflecting surface back to the object surface, and forms a ghost image on the object surface on the opposite side, with respect to the optical axis of the scanning lens system, to the side the beam spot is formed.
According to another aspect of the invention, the scanning optical system may be configured such that the rotation axis is inclined with respect to the auxiliary scanning direction to displace the beam spot in the auxiliary scanning direction, and the light beam incident on one of the reflecting surfaces is inclined with respect to a main scanning plane to displace the beam in a direction, along said auxiliary scanning direction, which is the same as a direction in which the beam spot is displaced due to the inclination of the rotation axis, the main scanning plane is parallel to the main scanning direction and including the optical axis of said scanning lens system.
In each aspect mentioned above, a light intercepting member may be provided on a passage of a light forming a ghost image on the object surface, but out of a passage of the light beam forming the beam spot on the object surface.
It is preferable that the light beam incident on the reflecting surface and the rotation axis of the polygonal mirror are inclined, respectively, such that a bow of a scan line, along which the light beam moves as the polygonal mirror rotates, caused by the inclination of the rotation axis of the polygonal mirror is canceled by a bow of the scan line caused by the inclination of the light beam incident on the reflecting surface.